1. Field of the Invention
The present invention relates to an optical rotary encoder for detecting angular position and velocity of a rotating wheel.
2. Description of Prior Art
An optical rotary encoder has been developed, which includes a slit plate secured to a shaft or a rotating wheel. A light emitting element and a plurality of light responsive elements are disposed in a manner such that the light beam emitted from the light emitting element toward the light responsive elements is interrupted by the slit plate. The slit plate is provided with a plurality of circumferentially aligned slits so that the light responsive elements periodically receive the light beam through the slits as the slit plate rotates in unison with the rotation of the shaft of the rotating wheel. Output signals derived from the light responsive elements are applied to a detection circuit for determining the angular position and velocity of the rotating wheel.
The conventional optical encoder includes at least two light responsive elements, which are offset from each other in the tangential direction by a distance equal to one half of the width of a slit, and are spaced from each other in the radial direction by a predetermined distance. Because of the displacement in the radial direction, it was difficult to adjust the levels of the output signals developed from the two light responsive elements. Furthermore, the light responsive elements occupy a considerably large space, which resulted in enlargement of the optical rotary encoder.
An example of an optical encoder is disclosed in U.S. Pat. No. 4,224,514, "Optical Encoder", issued on Sept. 23, 1980. The encoder disclosed in U.S. Pat. No. 4,224,514 ensures an accurate detection. However, the encoder disclosed in U.S. Pat. No. 4,224,514 requires two pairs of photosensors, each pair including two photosensors spaced from each other in the radial direction. Therefore, the encoder of U.S. Pat. No. 4,224,514 occupies a large space. Furthermore, the signal level adjustment is difficult.
In order to compensate for variations in the light beam intensity emitted from the light emitting element, in the conventional optical encoder, another light responsive element is disposed at a position which is suited for continuously receiving the light beam emitted from the light emitting element without being interrupted by the slit plate. An output signal derived from this light responsive element is used to determine a reference intensity level. This additional light responsive element further enlarges the optical encoder.
Furthermore, in the conventional optical encoder, the detection circuit is discrete from the substrate of the light responsive elements. Therefore, the system requires a cable for transmitting the output signals derived from the light responsive elements to the detection circuit. The cable makes the system large, and creates noises in the output signals derived from the light responsive elements.